A second expert on Britain’s 45-strong reservoirs panel has questioned the justification for draining Llanishen Reservoir, it has emerged.

Civil engineer Alan Warren raised fears about the work being done at the Cardiff beauty spot in a report compiled for the Environment Agency that has been released to campaigners.

He said that it was “not clear” why draining the site was “considered critical to improving reservoir safety” and said removing the water could create “new risks to reservoir safety”.

His report was released to the campaigning Reservoir Action Group after they applied under the Freedom of Information Act.

The campaigners hope his conclusions will put pressure on WS Atkins, the global engineering firm advising the site’s owner Western Power Distribution, to reconsider its crucial recommendation that the drain down is necessary and safe.

The firm’s engineer, Dr Andrew Hughes, said last year that in legal terms his advice was “a tablet of stone” that could not be changed.

But Cardiff North MP Jonathan Evans has invited Atkins chief executive Keith Clarke to visit the depleted reservoir to see what is being done in his firm’s name.

Mr Warren is now the second of the 45 British engineers who are qualified to inspect reservoirs who have said the drain down is not necessary and could risk damaging the reservoir’s long-term safety.

Fellow All Reservoirs Panel member Professor Chris Binnie, an independent expert employed by the campaigning Reservoir Action Group, also concluded there was no justification for draining the site in a report compiled last year.

Despite the concerns, neither the Environment Agency nor any other organisation is able to challenge the safety recommendation made by the official inspecting engineer employed through WS Atkins by Western Power Distribution.

A spokesman confirmed yesterday that the agency was still studying the US-owned firm’s application to discharge the remaining waters through a scour valve and planned to release its decision this month.

He confirmed that the agency’s role was to examine the potential damage to wildlife downstream of the reservoir.

Dr Hughes has been replaced as inspecting engineer by fellow Atkins employee Chris Owens who is proceeding with plans to drain the site and has submitted a proposed schedule of starting the draw down on July 19 and finishing on October 10.

Andrew Hill, the chairman of the Reservoir Action Group, said any dispute between four equally qualified engineers that had safety implications for a protected Victorian reservoir should be referred to the Institution of Civil Engineers.

He said: “I am particularly concerned that the engineer employed by the Environment Agency has come up with a view that what Atkins are doing at Llanishen is going to create doubts about reservoir safety.

“When you have got four engineers saying something different, then I think a responsible company like Atkins should seek advice from the Institution of Civil Engineers. I think that would be not only the professional but the reasonable thing to do.

“Alan Warren’s conclusions make it clear that there is no reason to drain the reservoir and what Western Power Distribution is doing are pure spoiling tactics.”

An Atkins spokesperson said that its engineer’s report was “a matter of legal record under the Reservoirs Act 1975 and cannot be altered”.

He said: “We recognise that there may be differences of opinion from leading industry experts on this matter, but believe that we are providing the most appropriate technical solution to our client.

“Atkins has earned a reputation for providing independent, high quality, technical advice in accordance with prevailing regulations. We are satisfied that we continue to provide the most appropriate, independent advice to our client on this issue.”